|

Wood Ash Database

– Wood for Energy -

Combustion

Grate boilerEriksson, L., Ingman, R. Anvisningar för konvertering av rosterpannor till fluidiserad bäddteknik. Värmeforsk.

In Sweden, Grate boilers have been used for a long time in industry as well as municipal energy facilities. The main use for these types of incinerators is when solid fuels are being used. For example, in Sweden most incinerators in the pulp and paper industry are of this type (about 90%).

Grate boilers consists of a grate on which the fuel bed is dried, pyrolyzed and incinerated (see Fig. 1 for more details on this process). The grate can be inclined as well as level, fixed as well as moveable. Combinations of fixed and movable types of grates also exist. The term movable grate implies that the grate rods are movable by different hydraulic systems in charge of propelling fuel.

Combustion bed

Fig 1. Schematic view of a combustion bed (Source: Combustion & FlameCombustion and Flame vol. 121)

Primary air needed in the incineration process is added beneath the grate, whereas secondary, tertiary and some times quaternary air used in the slutförbränning is added in different levels of the process. To be able to keep emission levels of carbon monoxide (CO) and unburned carbohydrates as low as possible, it is of great importance that the fuel bed is kept even, and that fuel is added at a speed which allows the fuel to be completely incinerated before it reaches the end of the grate. It is also essential that the added air is of correct stoichiometry, to prevent nitrogen oxide from forming.